mysql 編程筆記

要進行linux下的mysql的C編程，須要安裝mysql及mysql的開發包，ubuntu下直接apt-get install libmysql++安裝開發包。

#include <mysql.h>

相關函數:

MYSQL *mysql_init(MYSQL *);
//這裏稱之爲載入函數吧，返回的MYSQL指針要用到後續的函數中

int mysql_options(MYSQL *connection, enum option_to_set,const char *argument);
//設置MYSQL*的一些屬性，好比超時時間等

MYSQL *mysql_real_connect(MYSQL *connection,
                const char *server_host,
                const char *sql_user_name,
                const char *sql_password,
                const char *db_name,
                unsigned int port_number,//置0鏈接默認端口，通常爲3306
                const char *unix_socket_name,//NULL
                unsigned int flags);//無另外屬性時置0
//鏈接函數

void mysql_close(MYSQL *connection);
//關閉鏈接

unsigned int mysql_errno(MYSQL *connection);
//返回錯誤代碼

char *mysql_error(MYSQL *connection);

//返回錯誤信息

int mysql_query(MYSQL *connection, const char *query);
//執行sql語句

my_ulonglong mysql_affected_rows(MYSQL *connection);
//返回執行語句事後受影響的行數

MYSQL_RES *mysql_store_result(MYSQL *connection);
//返回執行結果，適用於數據量較小時

my_ulonglong mysql_num_rows(MYSQL_RES *result);
//返回上面函數返回結果的行數

MYSQL_ROW mysql_fetch_row(MYSQL_RES *result);
//抽取一條記錄，返回NULL時表示抽取完記錄或者錯誤

void mysql_data_seek(MYSQL_RES *result, my_ulonglong offset);
//調整數據位置，offset爲0時，下次調用mysql_fetch_row將返回result第一條記錄

MYSQL_ROW_OFFSET mysql_row_tell(MYSQL_RES *result);
//返回當前的位置

MYSQL_ROW_OFFSET mysql_row_seek(MYSQL_RES *result, MYSQL_ROW_OFFSET offset);
//移動數據位置，並返回先前的位置，能夠和上一個函數結合使用

void mysql_free_result(MYSQL_RES *result);
//釋放result空間

MYSQL_RES *mysql_use_result(MYSQL *connection);
//返回執行結果，適用於數據量較大時

unsigned int mysql_field_count(MYSQL *connection);
//返回查詢結果中的列數（column數）

MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result);
//得到查詢結果中的列名等信息（表頭信息）

例子：

建立測試數據庫
mysql> create database test;
建立用戶(已經建立)
mysql> grant all on test.* to mymotif@'localhost' identified by 'xxxxx';

sql文件：

--
-- Create the table children
--
CREATE TABLE children (
childno int(11) NOT NULL auto_increment,
fname varchar(30),
age int(11),
PRIMARY KEY (childno)
);
--
-- Populate the table ‘children’
--

INSERT INTO children(childno,fname,age) VALUES(1,'Jenny',21);
INSERT INTO children(childno,fname,age) VALUES(2,'Andrew',17);
INSERT INTO children(childno,fname,age) VALUES(3,'Gavin',8);
INSERT INTO children(childno,fname,age) VALUES(4,'Duncan',6);
INSERT INTO children(childno,fname,age) VALUES(5,'Emma',4);
INSERT INTO children(childno,fname,age) VALUES(6,'Alex',15);
INSERT INTO children(childno,fname,age) VALUES(7,'Adrian',9);

導入sql文件：

mysql -u mymotif --password=xxxx test<mysqlchildren.sql

導入後的狀況：

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| children |
+----------------+
1 row in set (0.00 sec)

mysql> select * from children;
+---------+--------+------+
| childno | fname | age |
+---------+--------+------+
| 1 | Jenny | 21 |
| 2 | Andrew | 17 |
| 3 | Gavin | 8 |
| 4 | Duncan | 6 |
| 5 | Emma | 4 |
| 6 | Alex | 15 |
| 7 | Adrian | 9 |
+---------+--------+------+
7 rows in set (0.00 sec)

C代碼:

/*
 *
 *   gcc mysqltest.c -o mysqltest  -I/opt/mysql/include -L/opt/mysql/lib -lmysqlclient -lpthread -lm -lrt -ldl
 *
 *   gcc mysqltest.c -o mysqltest  `mysql_config --cflags --libs`
 *
 * */

#include <stdio.h>
#include <stdlib.h>
#include <mysql.h>

MYSQL *mysql_main;
MYSQL_RES *res_ptr;
MYSQL_ROW sqlrow;
void display_header();
void display_row();
int main(int argc,char *argv[])
{
	int res;
	int first_row = 1;

	mysql_main = mysql_init(NULL);

	if(!mysql_main)
	{
		fprintf(stderr,"mysql_init failed\n");
		return EXIT_FAILURE;
	}
	mysql_main = mysql_real_connect(mysql_main,"localhost","mymotif","wxwpxh","test",0,NULL,0);
	if(mysql_main)
	{
		printf("Connection success:\n");
		res = mysql_query(mysql_main,"SELECT childno,fname,age FROM children WHERE age>5");
		if(res)
		{
			fprintf(stderr,"SELECT error: %s\n",mysql_error(mysql_main));
		}
		else
		{
			res_ptr = mysql_use_result(mysql_main);
			if(res_ptr)
			{
				while((sqlrow = mysql_fetch_row(res_ptr)))
				{
					if(first_row)
					{
						display_header();
						first_row = 0;
					}
					display_row();
				}
			}
		}
	}
	else
	{
		printf("Connection failed\n");
	}
	mysql_close(mysql_main);
	return EXIT_SUCCESS;
}
void display_header()
{
	MYSQL_FIELD *field_ptr;
	printf("Column details:\n");
	while((field_ptr = mysql_fetch_field(res_ptr))!=NULL)
	{
		printf("\t Name: %s\n",field_ptr->name);
		printf("\t Type: ");
		if(IS_NUM(field_ptr->type))
		{
			printf("Numeric filed\n");
		}
		else
		{
			switch(field_ptr->type)
			{
				case FIELD_TYPE_VAR_STRING:
					printf("VARCHAR\n");
					break;
				case FIELD_TYPE_LONG:
					printf("LONG\n");
					break;
				default:
					printf("Type is %d,check in mysql_com.h\n",field_ptr->type);
			}
		}
		printf("\t MAX width %ld\n",field_ptr->length);
		if(field_ptr->flags&AUTO_INCREMENT_FLAG)
		  printf("\t Auto increments\n");
		printf("\n");
	}
}
void display_row()
{
	unsigned int field_count;

	field_count = 0;
	while(field_count<mysql_field_count(mysql_main))
	{
		if(sqlrow[field_count]) printf("%s ",sqlrow[field_count]);
		else printf("NULL");
		field_count++;
	}
	printf("\n");
}

$ ./mysqltest 
Connection success:
Column details:
     Name: childno
     Type: Numeric filed
     MAX width 11
     Auto increments

     Name: fname
     Type: VARCHAR
     MAX width 30

     Name: age
     Type: Numeric filed
     MAX width 11

1 Jenny 21 
2 Andrew 17 
3 Gavin 8 
4 Duncan 6 
6 Alex 15 
7 Adrian 9 

注意事項：

當mysql用編譯源碼或二進制方式安裝時(非apt-get install)，而系統又經過apt安裝了mysql客戶端(因爲依賴關係)，這時連接mysqlclient 會連到apt安裝的那個庫：

$ ldd mysqltest
    linux-vdso.so.1 =>  (0x00007ffc5a9a0000)
    libmysqlclient.so.20 => /usr/lib/x86_64-linux-gnu/libmysqlclient.so.20 (0x00007f9c0d5e6000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9c0d21c000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f9c0cfff000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f9c0cdfb000)
    librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f9c0cbf2000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f9c0c86a000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f9c0c561000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f9c0c349000)

/usr/lib/x86_64-linux-gnu/libmysqlclient.so.20 -> /usr/lib/x86_64-linux-gnu/libmysqlclient.so.20.3.2

編譯安裝的服務器和客戶端與apt安裝客戶端socket文件不一致：編譯安裝的是/tmp/mysql.sock

apt安裝客戶端會鏈接/var/run/mysqld/mysqld.sock。顯然/var/run/mysqld/mysqld.sock是不存在的。

mysql_real_connect(mysql_main,"localhost","mymotif","wxwpxh","test",0,NULL,0)，用localhost（unix socket方式）是連不上服務器，提示：Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)。用127.0.0.1是能夠的（採用tcp方式）。

解決辦法：

把/usr/lib/x86_64-linux-gnu/libmysqlclient.so.20鏈接到服務器的那個庫。

 sudo ln -s /opt/mysql-5.7.14-linux-glibc2.5-x86_64/lib/libmysqlclient.so.20.3.1  /usr/lib/x86_64-linux-gnu/libmysqlclient.so.20